Magnetic storage having discrete elements with quantized magnetic moments

ABSTRACT

A magnetic storage includes a non-magnetic substrate. A plurality of discrete single magnetic domain elements formed of a magnetic material separated by nonmagnetic materials are carried on the non-magnetic substrate. Each single magnetic domain element has the same size, shape and has, without an external magnetic field, two quantized magnetization values. The two magnetization values are of substantially equal magnitude but of differing vector directions. The plurality of single domain elements are adapted for magnetic storage of information based upon direction of the magnetization vector. Each single magnetic domain element is used to store a bit of binary information. Writing each bit becomes to flip the quantified magnetic moment directions. Each bit can be tracked individually. The switching field of each bit can be controlled by controlling the size and shape anisotropy of each bit. Methods of fabricating the magnetic storage medium include obtaining the non-magnetic substrate and forming the plurality of single magnetic domain elements on the substrate.

This invention was partially sponsored by the United States Governmentunder Grant No. N00014-93-1-0648 awarded by the Advance ResearchProjects Agency and Grant No. N00014-93-0256 awarded by the Office ofNaval Research. The Government has certain rights in the invention.

This is a division of application Ser. No. 08/448,807, filed May 24,1995, now U.S. Pat. No. 5,820,769.

BACKGROUND OF THE INVENTION

The present invention relates to storing information. More specifically,the present invention relates to a magnetic storage.

In a conventional magnetic disk storage, information is stored in acontinuous magnetic thin film that is over a rigid nonmagnetic disk.Each bit of information is stored by magnetizing a small area on thethin magnetic film using a write head that will provide a suitablemagnetic field. The magnetic moment, the area and the location of thatsmall area present a bit of binary information, and they must be definedprecisely to allow a magnetic sensor, called a read head, to retrievethe written information.

The conventional magnetic disk storage suffers several drawbacks thathinder realization of ultrahigh density storage. First, the magneticmoments of a continuous film have an infinite number of possibilities.Therefore, the write head must write very precisely in defining themagnetic moment, the location, and the area of each bit on the magneticthin film. A slight error in doing so will not only create the error inthe bit, but also could miswrite the neighboring bits, causing errors inreading. Second, a continuous film is very good in linking exchangeinteraction and magnetostatic interaction that are between the bits.When the bits are very close, writing of one bit could lead to writingof its neighbors because of the exchange interaction and magnetostaticinteraction between the bits. Thirdly, the continuous magnetic filmmakes many bits have no physical boundaries between them, making thereading and writing in a blind fashion. This means that the location ofeach bit is found by calculating the movements of the disk and the writeor read heads, instead of physically sensing the actual bit location.Fourth, the continuous magnetic film also makes the boundary of two bitswith different magnetization ragged, creating noise in reading.

As demand for more information continues to grow, the need for highdensity data storage will keep increasing. To achieve ultrahigh densitymagnetic storage, the drawbacks of the conventional magnetic storagementioned above must be overcome.

SUMMARY OF THE INVENTION

The present invention presents a new paradigm in magnetic storage andits fabrication processes.

It is an objective of the present invention to use a new paradigm toovercome the drawbacks of previous magnetic storage and to achieveultrahigh storage density.

In the present invention, the continuous magnetic film used in aconventional magnetic disk is abandoned. In its place, the presentinvention uses a plurality of discrete elements of magnetic materials.Each discrete magnetic element is separated from other elements bynonmagnetic materials; the spacing is large enough that exchangeinteraction between two neighboring elements is either greatly reducedor eliminated. Each magnetic element has the same size and the sameshape, and is made of the same magnetic materials as the other elements.The elements are regularly arranged on the substrate. Each magneticelement has a small size and a preferred shape anisotropy so that,without an external magnetic field, the magnetic moments of eachdiscrete magnetic element will be automatically aligned to one axis ofthe element. This means that the magnetic moments of each discretemagnetic element is quantized and has only two states: the same inmagnitude but in two opposite directions. Such a discrete magneticelement is called a single magnetic domain element. The size, area andlocation of each bit were predetermined in the fabrication. Eachdirection of the quantized magnetic moments of a single magnetic domainelement is used to represent one value of a binary bit. A writingoperation in this invention is simply to flip the magnetic momentdirection of the single magnetic domain element. A reading operation inthis invention is to sense the quantized magnetic moments. The long axisof each element, and therefore of their magnetization, can be parallelto the surface of the medium, i.e., longitudinal recording, orperpendicular to the surface, i.e., perpendicular recording.

The magnetic storage consists of magnetic storage medium, write heads,and read heads.

The magnetic storage media are fabricated by obtaining a non-magneticsubstrate. Single magnetic domain elements are formed on thenon-magnetic substrate. In one embodiment, the single-domain elementsare adapted for vertical recording.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B show a magnetic storage medium system.

FIGS. 2A through 2D show steps in accordance with one method of makingthe storage medium of FIG. 1.

FIG. 3 is an SEM micrograph of a single magnetic domain pillar array.

FIGS. 4A through 4D show another method of forming the magnetic storagemedium of FIG. 1.

FIGS. 5A through 5F show another method of forming the magnetic storagemedium of FIG. 1.

FIGS. 6A through 6D show steps in accordance with another embodiment.

FIG. 7 is a diagram showing a relationship between switching fieldstrength (Oe) versus bar width (nm) for a nickel bar of 1 μm length and35 μm thickness.

FIG. 8 is a cutaway perspective view of a storage medium adapted forlongitudinal recording.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As used herein, the term "single magnetic domain" refers to magneticmoments of a magnetic element which automatically align in one directionin the absence of an external magnetic field. However, the magneticelement of a single magnetic domain can be made of either single crystalor polycrystal or amorphous materials.

FIGS. 1A-1B show magnetic storage medium (disk) 10 in accordance withone aspect of the invention. Disk 10 includes surface 12 which storesmagnetically encoded information. FIGS. 1A-1B also show an enlargedportion of disk 10 having non-magnetic substrate 14, single magneticdomain pillars 16 and non-magnetic material 18. Pillars 16 are of shapeand dimension such that they have two quantized magnetization values.These two magnetizations are of opposite vector directions which areperpendicular to the disk surface but of similar vector magnitude. Aread/write head 19 is positioned over surface 12 for reading and writinginformation.

FIGS. 2A through 2D show steps used to fabricate magnetic storage medium10. A thin gold (or Ti) plating base 20 is deposited on a non-magneticsilicon substrate 22. A high resolution electron beam photoresist,polymethyl methacrylate (PMMA) 24, is spun onto substrate 22. Dependingupon the desired pillar height, the thickness of PMMA 24 is typically130 nm; however, 720 nm thick PMMA 24 may also be used. Dot arrays 26with diameters from 35 nm to 40 nm and spacings from 50 nm to 1000 nmare exposed in the PMMA using a high resolution electron beamlithography system with a beam diameter of 4 nm. The exposed PMMA 24 isthen developed in a cellosolve and methanol solution creating a templatefor the electroplating process, as shown in FIG. 2. The sample isimmersed in a nickel sulfamate type plating bath and nickel iselectroplated into template openings 26 until the nickel thickness isabout the template thickness. This forms Ni pillars 28, shown in FIG.2C. The plating rate, which is a function of plating current, templatediameter and template thickness, is calibrated for about 45 nm/min.After electroplating, the PMMA template 24 is removed as shown in FIG.2D.

After fabrication, pillars 28 were examined using a scanning electronmicroscope (SEM) to verify the pillar dimensions. The resulting nickelpillars 28 were uniform and had the desired shape anisotropy. FIG. 3shows an SEM micrograph of a pillar array having a diameter of 35 nm, aheight of 120 nm and therefore an aspect ratio of 3.4. The pillar arrayhas a period (spacing) of 100 nm, and thus has a magnetic storagedensity of 65 Gbits/in² which is two orders of magnitude higher thantypical state-of-the-art storage. The pillars have a cylindrical shapewith very smooth side walls.

FIGS. 4A through 4D show steps in another method of forming singlemagnetic domain elements. In FIG. 4A, a non-magnetic substrate 40 haslayers of gold (or Ti) 42, SiO₂ 44 and PMMA 46 deposited thereon. InFIG. 4B, PMMA layer 46 has been exposed using E-beam lithography and wasdeveloped. Next, using PMMA as an etch mask, the SiO₂ layer 44 issubjected to reactive ion etching, forming cavities in SiO₂ layer 44.Following the reactive ion etch, PMMA layer 46 is chemically strippedand nickel 48 is electroplated onto the substrate through the cavities,as shown in FIG. 4D. Nickel 48 includes "bumps" where it has grown aboveSiO₂ layer 44. Following the deposition of nickel layer 48, the surfaceof the substrate is polished chemically and mechanically with a diamondslurry polish, for example, into a smooth surface with variations lessthan 5 Å. This yields disk 10 as shown in the inset in FIG. 1.

FIGS. 5A through 5F show steps in accordance with another method offorming single magnetic domain elements. In FIG. 5A, non-magneticsubstrate 50 has plating base 52, SiO₂ layer 54, chrome layer 56 andPMMA layer 58 deposited thereon. As shown in FIG. 5B, layer 58 isexposed to electron beam lithography and developed to form viastherethrough. Using the PMMA as a mask, a chrome etch is applied to formvias 60 in chrome layer 56, shown in FIG. 5C. Vias 60 are extended toplating base 52 in FIG. 5D using a reactive ion etching technique or ionmilling. The PMMA layer 58 and chrome layer 56 are removed chemically. Anickel electroplating step forms magnetic elements 62 shown in FIG. 5E.At FIG. 5F, the substrate has been exposed to a chemical or mechanicalpolishing process in which the surfaces of element 62 are lapped backand are congruent with the surface of layer 56. This forms the flatsurface shown in FIG. 5F which includes single magnetic domain element62 formed therein.

FIGS. 6A through 6D show another method of forming the storage mediumshown in FIG. 1. In FIG. 6A, a polymer layer 70 covers a non-magneticsubstrate 72. Opposite polymer layer 70 is mold 74 which includespillars 76. Mold 74 is moved toward polymer layer 70 and forms animprint on layer 70. This imprint forms a plurality of recesses (orvias) 78 as shown in FIG. 6B. To form this imprint, polymer layer 70should be at a temperature which allows pillars 76 to form recesses 78.Typically, this temperature will be slightly below the meltingtemperature of polymer layer 70. In one embodiment, pillars 76 of mold74 are formed of glass. Layer 70 is then exposed to a chemical vapordeposition process (CVD) in which magnetic material 80 such as nickel isdeposited as shown in FIG. 6C. Material 80 flows into recesses 78.Material 80 is then exposed to a chemical or mechanical polishing stepin which material 80 is removed from the surface to expose layer 70.This leaves a substantially flat surface for layer 70 containing singlemagnetic domain elements 82. The method described in the steps in FIGS.6A through 6D is well suited for large scale fabrication.

It should be understood by those skilled in the art that any appropriatefabrication technique may be used. Another example includes forming amask in material, such as photoresist, using a mold. Then the imprint ofthe molding will be transferred to the non-magnetic material by etchingtechniques.

If each pillar is used to store one bit of information, such a nanoscalepillar array storage has a rather different paradigm than theconventional magnetic storage. In conventional storage, each bit ofinformation is stored over a number of magnetic grains in a continuousmagnetic film which have a broad distribution in grain size, spacing andmagnetization direction. These distributions will result in thevariation of the total magnetization of each bit stored and give rise tonoise in reading. In the single domain pillar array, on the other hand,each bit is stored in a pillar which has only two quantizedmagnetization values: up or down in direction but equal in magnitude.Therefore, noise for each bit is small.

The magnetic field needed to switch the direction of magnetic moment ofeach discrete element can be controlled by controlling the size andshape anisotropy of each element. FIG. 7 is a graph of switch fieldstrength versus bar width for a nickel pillar of 1 μm length and 35 μmheight, in accordance with one embodiment.

FIG. 8 is a cross-sectional perspective view of magnetic storage medium100 in accordance with another embodiment. Storage medium 100 is adaptedfor horizontal or lateral recording. Medium 100 includes non-magneticsubstrate 102 carrying non-magnetic material 104. Material 104 carries aplurality of single magnetic domain bars 106. Bars 106 are adapted forhorizontal recording. The magnetization vectors of bars 106 are parallelwith the long axis of bars 106 and are capable of having two discretestates, as discussed above. Bars 106 are formed using any of theprocesses discussed herein.

Methods which may be able to characterize the nanoscale magnetic pillarsinclude magnetic force microscopy (MFM) scanning electron microscopywith polarization analysis (SEMPA) and magnetooptical Kerr effectmicroscopy (MOKE). MFM measures the magnetic field gradient using a tinymagnetic dipole moment which is scanning across the sample. SEMPAanalysis forms images by scanning a focused electron beam across asample and detecting the spin polarization of secondary electrons. Themagnitude and direction of the secondary electron's spin polarization isdirectly proportional to the magnitude and direction of themagnetization of the sample being scanned. MOKE analysis measures themagnetization of the pillars versus the magnetic field by detecting therotation of polarization state of light reflected from a ferromagneticsample. Measurement using a magnetic force microscope has shown thateach pillar is a single magnetic domain.

The advantages of disk 10 over the conventional disks are apparent.First, the writing process in disk 10 is greatly simplified, resultingin much lower noise and lower error rate and allowing much higherdensity. In disk 10, the writing process does not define the location,shape and magnetization value of a bit, but just simply flips thequantized magnetization orientation of a pre-patterned single domainmagnetic structure. The writing can be perfect, even though the headslightly deviates from the intended bit location and partially overlapswith other bits, as long as the head flips only the magnetization of theintended bit. In the conventional magnetic disk, the writing processmust define the location, shape and magnetization of a bit. If the headdeviates from the intended location, the head will write to part of theintended bit and part of the neighboring bits.

Secondly, the QMD can greatly reduce the crosstalk between theneighboring bits, offering much higher storage density. This is becausecrosstalk is due to the exchange and magnetostatic interaction bitslinked by continuous thin film Isolating each bit with non-magneticmaterial will greatly cut off these interactions and, therefore,crosstalk.

Thirdly, disk 10 can track every bit individually. In contrast, in aconventional disk each individual bit cannot be located. This is becausein disk 10 each bit is separated from others by nonmagnetic material,but in the conventional disk many bits are connected. Theindividual-bit-tracking ability allows precise positioning, lower errorrate and therefore ultra-high density storage.

Finally, reading in disk 10 is much less jittery than that in aconventional disk. The reason is that in a conventional disk theboundary between bits is ragged and not well defined, but in disk 10each bit is defined with nanometer precision (which can be less than agrain in size) and is well separated from each other.

Typical dimensions of the single magnetic domain element are that forvertical recording, it has a length of from about 0.1 μm to about 2.0μm, and a diameter of about 100 Å to about 5,000 Å, and that forlongitudinal (horizontal) recording, it has a width of about 50 Å toabout 0.5 μm, and a length of about 200 Å to about 1.0 μm. Suitablematerials include nickel, cobalt, iron and their alloys.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, the single domain element maybe arranged for horizontal (longitudinal) recording, or other materialsor fabrication techniques may be employed. Further, the cross section ofa quantum element can be other than the rectangular shape shown in thefigures.

What is claimed is:
 1. A magnetic storage system for storingmagnetically encoded information, comprising:a non-magnetic substrate; aplurality of discrete single-domain magnetic storage elements over thenon-magnetic substrate; wherein the plurality of discrete magneticelements are embedded in nonmagnetic materials, each discrete magneticelement having a shape anisotropy, spacing, and dimension to only allowits magnetization to one or the other of only two quantizedmagnetization values of differing magnetization vector directions and ofsubstantially equal magnetization vector magnitude in the absence of anexternal magnetic field, and each discrete magnetic element is used torepresent a binary bit of information based upon the magnetizationvector direction and substantially independent of magnetization vectormagnitude; a write head that flips the magnetization of one of theplurality of discrete magnetic elements to store a binary bit ofinformation; and a read head that reads the binary bit of information bysensing the quantized magnetic value from the one of the plurality ofdiscrete magnetic elements.
 2. The magnetic storage system of claim 1wherein each discrete magnetic element has a size and shapesubstantially equal to those of other of the plurality of discretemagnetic elements, and that is well controlled and precisely orientedduring fabrication.
 3. The magnetic storage system of claim 1 whereineach discrete magnetic element is made of magnetic materialssubstantially equal to those of other of the plurality of discretemagnetic elements.
 4. The magnetic storage system of claim 1 wherein theplurality of discrete magnetic elements comprise multiple layers offerromagnetic materials or alloys.
 5. The magnetic storage system ofclaim 1 wherein each discrete magnetic element has a small size andpreferred shape anisotropy such that each of the discrete magneticelements is a single magnetic domain thus providing the two magneticmoments of the element which tend to align along a long axis of theelement in the absence of an external magnetic field.
 6. The magneticstorage system of claim 1 wherein each discrete magnetic element isseparated from other discrete magnetic elements by nonmagnetic materialsto greatly reduce the exchange interaction and magnetostatic interactionbetween the discrete magnetic elements.
 7. The magnetic storage systemof claim 6 wherein the spacing between two neighboring discrete magneticelements is larger than 5 nm to reduce the exchange interaction betweenthe two neighboring discrete magnetic elements.
 8. The magnetic storagesystem of claim 1 wherein the write head has a size either smaller orlarger than a size of the discrete magnetic elements.
 9. The magneticstorage system of claim 1 wherein the non-magnetic substrate has a shapeof either a disk, a rectangle or a square.
 10. The magnetic storagesystem of claim 1 wherein the plurality of single domain elements aresubstantially perpendicular to the non-magnetic substrate for verticalrecording.
 11. The magnetic storage system of claim 1 wherein theplurality of single domain elements are substantially parallel to asurface of the non-magnetic substrate for longitudinal recording. 12.The magnetic storage system of claim 1 wherein the plurality of discretemagnetic elements are vertical pillars.
 13. The magnetic storage systemof claim 1 wherein the plurality of discrete magnetic elements arehorizontal bars.
 14. The magnetic storage system of claim 1 wherein amagnitude of a magnetic field needed to switch the magnetizationdirection of the quantized magnetization of one of the plurality ofdiscrete magnetic elements is determined by the size and shape of thatelement.
 15. The magnetic storage system of claim 1 wherein a single bitof information is stored using a single discrete single domain magneticelement.
 16. The magnetic storage system of claim 1 wherein a single bitof information is stored using a plurality of discrete single domainmagnetic elements.
 17. The magnetic storage system of claim 1 whereinthe plurality of discrete single-domain magnetic elements and thenonmagnetic materials together comprise a magnetic medium, and wherein asurface of the magnetic medium is substantially smooth.
 18. The magneticstorage system of claim 1 wherein the plurality of discretesingle-domain magnetic elements and the nonmagnetic materials togethercomprise a magnetic medium, and wherein a top surface of the magneticmedium includes topological variations.
 19. The magnetic storage systemof claim 1 including non-magnetic material between the plurality ofsingle domain magnetic elements which provides a variation in themagnetic field between the bits whereby the location of each element canbe tracked individually.
 20. A magnetic storage system for storingmagnetically encoded information, comprising:a substrate; a nonmagneticmaterial deposited on the substrate; a plurality of discretesingle-domain magnetic storage elements; wherein the plurality ofdiscrete single-domain magnetic storage elements are embedded in thenonmagnetic material, each single-domain magnetic storage element havingone of only two quantized magnetization moments of differingmagnetization vector directions and of substantially equal magnitude inthe absence of an external magnetic field; a write head that sets themagnetization vector direction of one or more of the plurality ofdiscrete single-domain magnetic storage elements in order to storebinary information; and a read head that reads the binary information bysensing the quantized magnetic moments from one or more of the pluralityof discrete single-domain magnetic storage elements.
 21. A magneticstorage medium for storing magnetically encoded information,comprising:a substrate; a nonmagnetic material deposited on thesubstrate; and a plurality of discrete single-magnetic-domain storageelements; wherein the plurality of discrete single-magnetic-domainstorage elements are embedded in the nonmagnetic material, eachsingle-magnetic-domain storage element having one of only two quantizedmagnetization moments of differing magnetization vector directions andof substantially equal magnitude in the absence of an external magneticfield.
 22. The magnetic storage medium of claim 21, wherein eachdiscrete single-magnetic-domain storage element has a size and shapesubstantially equal to the size and shape of other of the plurality ofdiscrete single-magnetic-domain storage elements.
 23. The magneticstorage medium of claim 21, wherein the plurality of discretesingle-magnetic-domain storage elements include a plurality of layers offerromagnetic materials or their alloys.
 24. The magnetic storage mediumof claim 21, wherein each discrete single-magnetic-domain storageelement has a small size and preferred shape anisotropy such that eachof the discrete single-magnetic-domain storage elements is a singlemagnetic domain thus providing the two magnetic moments of the storageelement which tend to align along a long axis of the storage element inthe absence of an external magnetic field.
 25. The magnetic storagemedium of claim 21, wherein a minimum spacing between any twoneighboring discrete single-magnetic-domain storage elements is largerthan about 5 nm in order to reduce the exchange interaction between thetwo neighboring discrete single-magnetic-domain storage elements. 26.The magnetic storage medium of claim 21, wherein the plurality ofdiscrete single-magnetic-domain storage elements are pillars each havingtheir longest axis oriented perpendicular to a surface of the substrateclosest to the plurality of discrete single-magnetic-domain storageelements.
 27. The magnetic storage medium of claim 21, wherein theplurality of discrete single-magnetic-domain storage elements are barseach having their longest axis oriented parallel to a surface of thesubstrate closest to the plurality of discrete single-magnetic-domainstorage elements.
 28. The magnetic storage medium of claim 21, whereineach single bit of information is stored using a plurality of discretesingle-magnetic-domain storage elements.
 29. The magnetic storage mediumof claim 21, wherein the plurality of discrete single-magnetic-domainstorage elements and the nonmagnetic materials together comprise amagnetic medium, and wherein a surface of the magnetic medium issubstantially smooth.
 30. The magnetic storage medium of claim 21,including non-magnetic material between the plurality of discretesingle-magnetic-domain storage elements which provides a variation inthe magnetic field between the bits that allows the location of storageelements to be tracked.
 31. The magnetic storage medium of claim 21,wherein each one of the plurality of discrete single-magnetic-domainstorage elements is located at a predetermined location in a pillararray such that successive bits of information are stored in successiveelements in the array on a one-to-one basis.